Refactor SimpleSAML_XHTML_Template

A template now has a translator-object, SimpleSAML_Locale_Translate.
This handles the existing translation system.

The translator has a language-object, SimpleSAML_Locale_Language. This
handles discovery and switching of locale.

SimpleSAML_XHTML_Template has several methods that are wrapped
translator-methods:

t()
isLanguageRTL()
getLanguageList()
getTranslation()

This way it isn't necessary to change much of the existing system.